Average atomic mass

Mahopac High School
Average Atomic Mass
Adapted from “It’s Beanium”
You have 20 of helium atoms sitting in a box. You are asked to find the average
Most of the atoms' masses are 4 amu’s (4He), but 2 are 3 amu’s (3He). How
would you find the average atomic mass?
First add up all the helium atoms with a mass of 4 amu’s (18 of them), and then
add to that all of the helium atoms with a mass of 3 amu’s (2 of them).
(18 heliums x 4 amu’s) + (2 heliums x 3 amu’s) = 78 amu. This is the sum of all
their masses.
To find the average mass, divide by the total mass by the total number of atoms
78 amu / 20 total heliums = 3.9 amu per helium.
The average mass of the helium atoms in your box is 3.9.
What if I didn't tell you the number of atoms in the box, and I only told you that
10% are 3He, and the rest are 4He. How could still calculate the average mass?
1) Sort the beanium atoms into groups, each group representing a different
2) Record the total number of atoms (beans) in your sample, as well as the
number of atoms (beans) of each isotope on your data sheet.
3) Find the total mass of each bean isotope.
4) Calculate the average atomic mass of each bean isotope.
5) Determine the percent abundance of each bean isotope in the sample.
6) Determine the atomic mass for “beanium’ based on your abundances for each
bean isotope and their atomic masses.
Bean 1
Mass of a single
% abundance
Bean 2
Bean 3
Data Table:
Sketch of
# of (atoms
Total mass of Mass of a
each bean
of) each bean each bean
single (atom
of) bean
% abundance
Bean 1
Bean 2
Bean 3
Post Lab Questions:
1) Calculate the average atomic mass of potassium if the abundance and atomic
masses making up its naturally occurring isotopes are:
% abundance
Mass of isotope
2) The atomic mass of copper is 63.540 amu. It is composed of 2 isotopes;
Cu-63 and Cu-65, with atomic masses of 62.390 amu and 64.928 amu
respectively. What is the relative abundance of these isotopes in naturally
occurring samples?
3) Write a short summary explaining how this lab illustrates the concept of
average atomic mass.